When integrated circuits are fabricated, isolated active-region regions are created and connected through electrical interconnect paths to implement the desired circuit function. Wherever a connection is needed, an opening in the active-region must be provided to allow such contacts to occur. In multilevel-interconnect structures, the contact holes are openings into the active-regions and openings called vias in the intermetal dielectric layers, which allow contacts to be made between adjacent metal layers.
FIG. 1 is a cross-sectional view of a contact hole 10 over an active-region 12. The contact hole has been formed through a layer of glass 14 and a layer of photoresist 16. The process for forming the contact hole 10 begins by covering the active-region 12 with the layer of glass 14 followed by the layer of photoresist 16. The location of the contact hole 10 is patterned by using a stepper device to expose the photoresist 16 through a mask to form the opening in the photoresist 16 at the desired locations. A plasma etcher then etches the glass 14 down to the active-region 12 through the opening in the photoresist 16 to form the contact hole 10. To enable proper contact between the interconnect and the active-region 12, the diameter of the contact hole 10 must be within a certain tolerance, e.g., 0.5 to 0.75 .mu.m, and the sides of the contact hole 10 should be substantially straight and clear of debris.
During contact hole formation, however, process variations may be encountered that result in malformed contact holes that prevent the interconnects from making proper contact with the device. During deposition of the photoresist 16, for instance, the track system that deposits the photoresist 16 may vary the thickness and concentration of the photoresist 16. When the photoresist 16 is then exposed to light by the stepper device, the varying photoresist thickness may cause improper focusing of the light during the exposure. The stepper's focus may also drift independent of the photoresist.
Regardless of the cause, focus drift causes contact hole diameter to become too large or two small. In addition, an out of focus exposure may create a contact hole that has sloped sidewalls because the exposure failed to clear all the photoresist in the opening down to the glass layer 14. Sloped sidewalls at the bottom of the opening will cause the diameter of the hole etched into the glass to be too small for adequate interconnect contact.
Even with an in focus exposure that forms a proper opening in the photoresist 16, there is no guarantee that the plasma etcher used to etch the glass will result in correct contact hole formation, because the plasma etcher is also subject to variations in power, temperature, and gas flow for example.
Consequently, various types of process drifts encountered during contact hole formation may result in some contact holes being formed with inadequate diameters and/or sidewalls. After contact hole formation, therefore, the contact holes must be inspected. The conventional method for inspecting the contact holes typically involves two steps. The first step is to measure the critical dimension (CD), or diameter, of each contact hole using a top-down scanning electron microscope (SEM) tool, referred to as a CD-SEM. The current CD-SEM tools have integrated pattern recognition capabilities to produce a waveform of the contact hole at a location of interest from which the diameter of the hole can be measured.
FIGS. 2A-2C are diagrams illustrating top views of several contact hole images and corresponding waveforms produced by the CD-SEM. FIG. 2A shows a contact hole printed at a stepper's center of focus (COF). FIG. 2B shows a contact hole printed at -0.6 .mu.m from the COF, and FIG. 2C shows a contact hole printed +0.6 .mu.m from the COF. As shown, each waveform is a plot of intensity values of the contact hole image. The light pixels in the image outlining the diameter of the hole generate the highest intensity values, or peaks, on the waveform. The intensity values of the waveform or their derivatives are then analyzed by an algorithm that measures the distance between the two points in the waveform, e.g., the two maximum peaks, to determine the CD of the corresponding contact hole. Traditionally, the algorithm for measuring diameters is chosen based on match-up to final product size and reproducibility, so they are by design insensitive to process variations.
Although such CD measurement is useful for insuring that the contact holes are within diameter tolerance, the CD measurement does not detect the presence of sloped sidewalls, hence the need for the second inspection step. In the second inspection step, the contact holes are manually inspected for sloped sidewalls by viewing into the holes through a scanning electron microscope, which increases both the time and cost of fabrication.
According, what is needed is an improved method for fabricating and inspecting integrated circuit contact holes, and more particularly, an improved method for detecting contact holes having sloped sidewalls, and in the extreme case, uncleared contact holes. The present invention addresses such a need.